Gari solida is an economically important sub tidal bivalve that inhabits shallow waters of Peru and Chile. Its respiration (RR) and filtration (FR) rates were analyzed in situ on individuals belonging to three ranges of size (45-55mm, 72-87mm and 92-97mm) in Paracas Bay. Experiments were conducted during the months of March 2007, August 2007 and February 2008 and consisted in the installation of twelve acrylic chambers, placed on the seabed at a depth of 4 m, fitted with electric submersible pumps for the recirculation of water, so the individuals could stayed in a closed system through a specific time. The RR and FR per gram of individual were about 4 times bigger in smaller individuals (RR = 0.22 mg h ̅ 1.g ̅ 1, FR = 0.38 L.h ̅ 1.g ̅ 1) compared to the larger size ranges (RR = 0.05 mg h ̅ 1.g ̅ 1, FR=0.08 L.h ̅ 1.g ̅ 1), which could be due to the increase in tissues of low metabolism as the animals grow. Furthermore, we found no relationship between the levels of dissolved oxygen (5 - 9 mg/L) and chlorophyll-a (2 5 μg/L) present during the evaluations and the physiological rates obtained, however, a seasonal change in the respiration rates was observed obtaining higher averages for the larger individuals during the winter (0.17 mg h ̅ 1.g ̅ 1) when compared with the summer (0.12 mg h ̅ 1.g ̅ 1). Finally, after analyzing the composition of phytoplankton on filtered water, G. solida showed a clear preference for phytoflagellates organisms (75%) in its diet.